1. Field of the Invention
The present invention relates, generally, to devices and methods for fine tuning the optical path length in etalon cavities and, in preferred embodiments, to devices and methods for fine tuning the optical path length in etalon cavities by varying the relative gas mixture in gas-gap etalons or UV-exposing intra-cavity layer(s) in solid etalons or gas-gap+glass etalons.
2. Description of Related Art
An etalon is an optically transparent medium sandwiched between two mirrors. In practice, the cavity formed between the two mirrors can be a parallel plate of glass with mirror coatings on both sides, or two parallel mirrors separated by a distance. Such an etalon is also known as Fabry-Perot etalon if both mirrors are identical. In the case when one mirror has a 100% reflectivity, such etalons are known as Gires-Tournois etalons. The optical path length (OPL) in the cavity of an etalon is defined as                               OPL          =                                    ∑                              j                =                1                            J                        ⁢                          xe2x80x83                        ⁢                                          L                j                            ⁢                              n                j                                                    ,                            (        1        )            
where Lj is the physical length of a path and nj is the refractive index in that length. This concept is diagrammed in FIG. 1 for an example etalon having two sections. In FIG. 1, a first cavity section 101 has a physical thickness L1 and a refractive index n1, while a second cavity section 102 has a physical thickness L2 and a refractive index n2. The fine-tuning of an OPL can be achieved by adjusting either L or n for a given OPL section. Previous similar solutions include electro-optic modulation of n, magneto-optic modulation of n, piezo-electric modulation of L, thermal modulation of n and/or L, and the like. However, conventional methods either do not allow for fine tuning of an OPL, or for permanent fine-tuning of an OPL in-situ.
It is an advantage of embodiments of the present invention to provide a device and method for fine-tuning the optical path length in etalon cavities.
The above-described and other advantages are accomplished by slight variations of the cavity""s index of refraction. Such index of refraction variations are accomplished by varying the relative gas mixture in gas-gap etalons or UV (or other)-exposing intra-cavity layer(s) in solid etalons or gas-gap+solid etalons.